1. Field of the Invention
This invention generally relates to batteries and, more particularly, to a metal-ion battery with large interstitial spacings.
2. Description of the Related Art
A battery is an electrochemical cell through which chemical energy and electric energy can be converted back and forth. The energy density of a battery is determined by its voltage and charge capacity. Lithium has the most negative potential of −3.04 V vs. H2/H+, and has the highest gravimetric capacity of 3860 milli-amp-hours per gram (mAh/g). Due to their high energy densities, lithium-ion batteries have led the portable electronics revolution. However, the high cost of lithium metal renders doubtful the commercialization of lithium batteries as large scale energy storage devices. Further, the demand for lithium and its reserve as a mineral have raised the need to build other types metal-ion batteries as an alternative.
Lithium-ion (Li-ion) batteries employ lithium storage compounds as the positive (cathode) and negative (anode) electrode materials. As a battery is cycled, lithium ions (Li+) exchange between the positive and negative electrodes. Li-ion batteries have been referred to as rocking chair batteries because the lithium ions “rock” back and forth between the positive and negative electrodes as the cells are charged and discharged. The positive electrode (cathode) materials is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material having a tunneled structure, such as lithium manganese oxide (LiMn2O4), on an aluminum current collector. The negative electrode (anode) material is typically a graphitic carbon, also a layered material, on a copper current collector. In the charge-discharge process, lithium ions are inserted into, or extracted from interstitial spaces of the active materials.
Similar to the lithium-ion batteries, metal-ion batteries use the metal-ion host compounds as their electrode materials in which metal-ions can move easily and reversibly. As for a Li+-ion, it has the smallest radius of all metal ions and is compatible with the interstitial spaces of many materials, such as the layered LiCoO2, olivine-structured LiFePO4, spinel-structured LiMn2O4, and so on. Other metal ions, such as Na+, K+, Mg2+, Al3+, Zn2+, etc., with large sizes, severely distort Li-based intercalation compounds and ruin their structures in several charge/discharge cycles. Therefore, new materials with large interstitial spaces would have to be used to host such metal-ions in a metal-ion battery.
FIG. 1 depicts the framework for an electrode material with large interstitial spaces in a metal-ion battery (prior art). It is inevitable that the large interstitial spaces in these materials readily absorb water molecules and impure ions, as shown. Although these open spaces are very suitable for the intercalation of metal-ions with large sizes, the water molecules and impure ions degrade the electrochemical performance. In this example, Prussian blue analogues (PBs) with cubic framework have open “zeolytic” lattices that permit Na+/K+ions to move easily and reversibly in the framework.
FIG. 2 demonstrates the crystal structure of Prussian blue and its analogues (prior art). Their general molecular formula is AM1M2(CN)6.zH2O, in which A is an alkali or alkaline-earth ion, and M1 and M2 are metal ions. The M1 and M2 metals are arranged in a three-dimensional checkerboard pattern, and shown in a two-dimensional pattern. The M1 ions are octahedrally coordinated to the nitrogen ends of the CN− groups, and the M2 ions to their carbon ends. The M1 and M2 ions are connected by the C≡N to form the Prussian blue framework with large interstitial spaces. The large interstitial sites may host the large sized alkali or alkaline-earth ions (A) and or zeolitic H2O. The ion channels connecting the interstitial sites are similar in size to solvated alkali ions such as sodium and potassium, allowing rapid transport of these ions throughout the lattice. Therefore, PB is a good choice for an electrode material in sodium/potassium-ion batteries. Nonetheless, thermogravimetric analysis (TG) suggests that every PB molecule contains four water molecules. The occupation of water and impure ions in these materials definitely reduces the spaces to host the metal-ions and leads to the reduced capacity of these electrode materials. Therefore, KCuFe(CN)6 has a theoretical capacity of 85.2 mAh/g, but its practical capacity is smaller than 60 mAh/g. In addition, water may react with the intercalated metal-ions and decrease the coulombic and energy efficiencies of the metal-ion batteries. Up to now, no method is reported to remove the water and impure ions from the large interstitial spaces of the hexacyanometallate electrode materials for metal-ions batteries. As a result, most metal-ions batteries with a hexacyanometallate electrode use an aqueous solution as an electrolyte. These batteries have small specific capacities and low voltages.
It would be advantageous if the aforementioned electrode forming problems could be addressed with a process to remove water and impure ions from the electrode material interstitial spaces. After such a process, the electrodes formed would be stable in a non-aqueous electrolyte, and metal-ion batteries made therefrom would have larger voltages and capacities.